Pilot-assisted, pressure relieving control valves, often called overcenter or holding valves, are used in a variety of fluid control applications. In one application, a valve of this type is used to control fluid flow to and from an actuator and to hydraulically lock the actuator in position when fluid flow is terminated. In this application, this type of valve prevents load induced "runaway" and provides a static overload relief function. In some commercially available valves, a check valve section is included as part of the valve and allows flow to proceed unimpeded, from the source to the actuator but prevents fluid flow from the actuator to the source until a predetermined system pressure is reached or until a predetermined pilot pressure is applied to a valve member in the valve.
In an application where the actuator is part of an operating system for a boom assembly, a pilot-assisted, pressure relieving valve is used to control the raising and lowering of the boom and more importantly, is used to lock the boom assembly in position when fluid flow is terminated. Specifically, in order to raise the boom assembly, fluid is directed to the boom actuator via the check valve section of the valve. As soon as flow terminates, the check valve operates to prevent return fluid flow from the actuator to the source until a pilot pressure is applied to a flow control element within the valve or until a predetermined system pressure is exceeded.
Another application for this type of control valve is in a hydrostatic drive for a vehicle. Those skilled in the art will recognize that a hydrostatic drive generally comprises a hydraulic pump for generating a source of pressurized fluid, a fluid motor connected to the drive wheels of the vehicle and conduits for conveying pressurized fluid from the pump to the fluid motor and vice versa. It is desirable to provide a deceleration function in the hydrostatic drive system so that vehicle motion will be retarded when the output of the fluid pump is reduced or terminated. As is well known, during vehicle deceleration, the fluid motor is driven by the vehicle wheels and generates an output fluid pressure. By restricting the return flow of fluid from the motor to the source or fluid pump, a deceleration force that resists rotation of the vehicle wheels will develop, thereby slowing the vehicle. It has been found that pilot-assisted, pressure relief valves disposed in the fluid conduits between the fluid pump and fluid motor could possibly provide the necessary flow resistance. In theory, these valves would prohibit flow from the fluid motor to the actuator until a predetermined pressure was reached or until a control or pilot pressure was applied.
It has been found however, that many commercially available control valves of this nature, do not work satisfactorily in applications where significant "back pressure" is encountered. It was found that in many prior valves of this type, system backpressure would be manifested as an increase in the effective relief setting of the valve.
Sequence control valves often include functional similarities with pilot-assisted, pressure relieving valves. Sequence valves are employed in fluid circuits to control the order of communication between a source of pressurized fluid and one or more fluid actuated devices. By construction, sequence valves usually operate as simple on/off valves that is, once a threshold pressure has been reached at the input port, the valve completely opens to allow fluid flow between input port and an output port. In general, the sequence valve will not throttle fluid flow as a function of input pressure. Pilot-assisted, pressure relieving valves on the other hand will open in proportion to the applied pilot pressure and therefore as the pilot pressure is reduced, flow through the valve will be throttled. Due to the operational differences between sequence and pilot-assisted, pressure relief valves, they have not generally been interchangeable in a given application. Specifically, it was found that the operation of many, if not most pilot-assisted relief valves, were detrimentally affected by system backpressure normally encountered in sequence valve environments.
In pilot-assisted valves it is also important that the pilot ratio not be excessive. Excessive pilot ratio causes the valve to be unduly sensitive and when used in control applications, i.e., the raising and lowering of a boom assembly, smooth gradual actuation of the hydraulic system is difficult to achieve by the operator. In some prior art pilot-assisted, pressure relieving valves it was found that although they were relatively unaffected by system backpressure, their configuration resulted in an excessive pilot ratio, i.e., 25:1 or more.